Field of the Invention
The present invention relates to an antireflection optical member, a method for manufacturing the optical member, and an optical film of the optical member and more particularly to an optical member having high antireflection performance from a visible region to a near-infrared region and a method for manufacturing the optical member.
Description of the Related Art
In an antireflection structure having a fine periodic structure having a pitch less than or equal to a wavelength in a visible light region, it is known that the formation of a fine periodic structure having an appropriate pitch and height results in high antireflection performance in a wide wavelength range.
A known method for forming a fine periodic structure includes the application of a film in which fine particles having a particle size less than or equal to the visible light wavelength are dispersed. A method for forming a fine periodic structure by patterning with a micromachining apparatus, such as an electron-beam lithography system, a laser interference exposure apparatus, a semiconductor exposure apparatus, or an etching apparatus, is known to enable the control of pitch and height and allows the formation of an antireflection fine periodic structure (Japanese Patent Laid-Open No. 50-70040).
In accordance with another method, boehmite, which is an aluminum oxide hydroxide, is grown on a substrate to produce an antireflection effect. In accordance with these methods, an aluminum oxide film formed by a liquid phase method (sol-gel method) is subjected to steam treatment or hot-water immersion treatment to form a fine periodic structure of crystals, for example, made of boehmite on its surface layer, thereby forming an antireflection film (see K. Tadanaga, N. Katata, and T. Minami: “Super-Water-Repellent Al2O3 Coating Films with High Transparency”, J. Am. Ceram. Soc., 80[4], 1040-42 (1997)).
When a substrate has a high refractive index, however, a sufficient antireflection effect cannot be produced with a fine periodic structure of aluminum oxide crystals alone. Thus, a layer having an intermediate refractive index between the refractive indexes of the substrate and the fine periodic structure is formed between the substrate and the fine periodic structure to improve the antireflection effect. In particular, a layer having an intermediate refractive index made of a transparent organic resin not only has an antireflection effect but also protects a glass substrate from damage caused by moisture or water vapor (see Japanese Patent Laid-Open No. 2008-233880).
In order to produce an antireflection film suitable for optical members made of a wide variety of glasses, such as high-refractive-index glasses, there is a demand for an optical thin film having an adjustable refractive index and small variations in film thickness and optical properties.
An antireflection porous film containing fine particles in its surface layer and a film having a textured structure containing aluminum oxide crystals are convenient and have high productivity and excellent optical performance. However, a large difference in refractive index between a substrate and these films results in an insufficient antireflection effect. A layer having an intermediate refractive index disposed between the substrate and the porous film or the film having the textured structure can improve the antireflection effect. However, the refractive index of the substrate varies greatly with the application of the substrate. Thus, there is a demand for a thin-film material that allows for a refractive index and a thickness suitable for the refractive index of the substrate. There is also a demand for an optical member having a high-performance antireflection film that has high in-plane uniformity.